The present invention relates to moisture-proofing electrical cables, particularly high-power cables having a synthetic insulation; and more particularly, the invention relates to a cable for three conductors with a common, metallic, corregated sleeve or envelope.
High-power cables generally are provided with a metal shield, being disposed over and above the insulation. The insulation may be covered with an outer, conductive, or more accurately, semiconductive layer; and the shield is provided on top of that layer. The shield is comprised, for example, of wires, metal ribbons, or the like, the material being, for example, copper. The metal shield as such has primarily an electrical function; however, German printed patent application No. 15 40 430 suggests already a combining of that function with a moisture-sealing function for preventing penetration of moisture into the insulation. The metal envelope is comprised in this instance of a metal ribbon which is longitudinally paid and folded around the insulated cable conductor. Overlapping ribbon edges are subsequently soldered in order to obtain the requisite sealing.
The shielded and moisture-proofed cable as per the German application No. 15 40 430 is disadvantaged by the fact that upon loading the cable and upon commensurate development of heat, the insulation expands more than the outer, closed metal envelope. The electrical insulation, after all, functions also as a terminal insulation so that the difference in radial expansion is not only the result of different coefficients of expansion, but the effective temperatures differ as well. Since the metal shield--tube--envelope is radially expandable to a limited extent only, one must expect the solder seam to rupture at any weak spot, or even over a large portion. Aside from these problems arising in the field and during use, the metal ribbon renders the cable rather stiff which interferes with the transport and reeling of the cable on drums as well as installation thereof.
One of the problems above has been avoided by corrugating the outer shield-envelope (German printed patent application No. 28 08 214). The cable as per that disclosure includes also features for compensating the difference in radial expansion of insulation and outer shield. The resulting cable does, indeed, avoid the outlined drawbacks. Still, special precautions are needed in those instances in which the cable is expected to experience significant abuse and wear, such as during installation in mines, or in underground sites generally, through rivers or lakes, or the like. The special precautions needed for these instances consist primarily in a particular armoring.